It shouldn't be an issue at all. If I'm reading your post right, you do not need the green border on the square, so cut it square (without the green borders), and then add the border you want, making it 20 1/2". So if you square it up to 18 inches, add a 1 1/2 border to all sides and you will end up with the 20 1/2" block you neeed. It is such a cute panel, I can hardly wait to see how you finish it. Happy Quilting.